The use of inhalation devices in the administration of medicaments, for example in bronchodilation therapy is well known. Such devices generally comprise a body or housing within which a medicament carrier is located. A known inhalation device has a medicament carrier in the form of an elongate blister strip containing a number of discrete doses of medicament in powder or tablet form. The elongate blister strip comprises a base sheet having pockets defined therein and a lid sheet provided thereto, wherein the base sheet and lid sheet are peelably separable to allow access to the contents of each pocket. Such devices typically contain a mechanism of accessing these doses comprising peeling means for peeling the lid sheet away from the base sheet. The medicament is thereby made available for delivery to the patient.
Suitable peeling means are positioned to peel apart a base sheet and a lid sheet of a pocket at an opening station of the device. The peeling means typically includes a (lid or base) sheet driver for pulling apart a lid sheet from a base sheet of a pocket that has been received at the opening station. In one aspect, the sheet driving means comprises a fixed-diameter wheel on which the (e.g. lid) sheet is wound, the wheel having an effective winding surface, the diameter of which increases as more (e.g. lid) sheet is wound about said wheel.
A problem encountered with the use of such a fixed-diameter wheel as the sheet driver for driving a sheet of a medicament carrier is that as the sheet winds up around the wheel the effective winding diameter of the wheel increases, and therefore its effective lateral pulling action (i.e. length of pull) also increases. This is problematic because it is desirable that on actuation, a definable pull action is experienced by the medicament carrier pocket at the opening station to ensure that a generally uniform indexing/opening effect is experienced by each pocket of the medicament carrier. In general terms, insufficient pull action will result in failure to open up the pocket whilst excess pull will put stress on the mechanical components and increase the force required to actuate the dispenser.
A solution to the above problem has been proposed in Applicant's PCT Patent Application No. WO 03/035509, in which compensating means are provided to compensate for any increase in the diameter of the effective winding surface of the wheel during use of the dispenser and thereby to ensure that said medicament carrier is uniformly indexed upon each actuation of said dispensing mechanism.
The Applicant has now found that a (lid or base) sheet driver having the form of a hub incorporating compensating means in the form of a central shaft-mounted torsion spring provides particularly effective compensation for this increase in diameter.